Preheat your oven to 350°F (175°C). Grease a 17x11 inch baking sheet and line it with parchment paper to ensure easy removal after baking.
In a saucepan, combine the cubed unsalted butter, water, and unsweet dark cocoa powder. Heat gently, stirring until smooth, then allow it to cool slightly.
In a mixing bowl, whisk together all-purpose flour, packed light brown sugar, espresso powder (if using), baking soda, and fine grain sea salt. Create a well in the center of the dry mix.
In a separate bowl, whisk together the buttermilk, large eggs, and vanilla extract. Pour this mixture into the cooled chocolate base and stir until well combined with the dry ingredients.
Spread the batter evenly in the prepared baking pan and bake for 18-20 minutes. A toothpick inserted into the center should come out clean. Allow the cake to cool before freezing it for 30 minutes to make assembly easier later.
Once cooled, trim the cake into three equal rectangles. Chill the layers while you prepare the fillings to help with easier assembly.
In a heatproof bowl, combine the heavy cream and semisweet chocolate chips. Heat in the microwave or over a double boiler until melted, then whisk until smooth. Let it cool to thicken.
In a large bowl, whip the remaining heavy cream until stiff peaks form. Gently fold this whipped cream into the cooled ganache for a light and airy texture. Refrigerate until you're ready to use it.
In another bowl, beat the softened cream cheese until creamy. Add the melted white chocolate, butter, lemon juice, confectioners' sugar, and a splash of vanilla extract, mixing until completely smooth.
On a serving platter, stack the layers of cake with a generous layer of chocolate mousse followed by a layer of the cream cheese filling. Repeat the process and refrigerate for at least one hour to set.
Melt additional chocolate chips with cream and a tablespoon of oil to create a silky ganache. Pour it over the chilled cake, ensuring it drips down the sides for an enticing effect.
Using any remaining ganache or a white glaze, drizzle over the top creatively. Pipe chocolate buttercream if desired, and finish with cookie wafers and elegant chocolate truffles for a beautiful presentation.
Refrigerate the entire cake until you are ready to slice and serve. This sweet masterpiece is best enjoyed after sitting at room temperature for a little while.